1. Field of the Invention
This invention relates to the field of strain-gauge amplifiers, and more particularly to a sensitive, differential bridge amplifier which employs a single temperature sensing element to provide offset voltage temperature compensation and gauge temperature compensation for wheatstone bridge type strain transducers or similar resistance bridge transducers.
2. Description of the Prior Art
Transducers are devices which convert one physical quantity (the stimulus) into another. A class of transducer is the environmental sensor which converts a physical quantity to be measured into an electrical, optical, or similar signal which is received by alarm or process control instruments. Many of these sensors, such as absolute pressure sensors, differential pressure sensors, and strain sensors are designed to convert from the stimulus to a resistance imbalance of a wheatstone bridge circuit. In FIG. 1, for example, the application of a stimulus, such as pressure, to a membrane with affixed or integral resistive elements can increase or decrease these resistance elements by the piezoresistive effect, thereby creating a bridge imbalance. The piezoresistive elements can be arranged so that this stimulus increases the value of resistors 2 and 5 and decreases the value of resistors 3 and 4. This imbalance will create a differential voltage output 6 to appear. Output voltage 6 is a function of the resistance values 2-5 and the excitation voltage 1, as follows: EQU Output Voltage=(Excitation Voltage) EQU [(R101/R103)-(R104/R102)]
In practical embodiments of such a bridge sensor, changes in the bridge resistances with applied stimulus are designed to be small in order to attain a linearity of response of the output voltage and the stimulus. Amplification of the resultant small bridge output voltages is therefore usually necessary to produce a signal of a strength needed to control alarm or process circuitry. The necessary signal strength is generally of the order of 4-20 mA for a current output and 0-1 V for a voltage output.
A major problem encountered with sensor/transducers of the type described is their undesirable response to temperature. Since the change of resistance with stimulus is designed to be small, temperature induced changes in the resistances of the bridge circuit can cause a large imbalance signal at a static stimulus. This is known as an offset, or zero, error. In addition, a temperature change is likely to cause a change in the coupling between the stimulus and the bridge resistors, leading to a calibration, or gauge, error; that is, greater or lesser stimulus would be required to generate a particular bridge imbalance at different temperatures. Both the offset and gauge errors can be approximated as linear functions of the sensor temperature.
To overcome this problem, attempts have been made at temperature compensation in strain-gauge amplifiers. In particular, R. Dell'Acqua et al. ("Characteristics and Performances of Thick-film Pressure Sensors for Automotive Applications", S.A.E. Paper 820319, Detroit, 1982) have used thermistors to achieve gauge and offset temperature compensation. The temperature sensing elements in their gauge are thermistors, which respond to temperature in a non-linear fashion, and are not effective over a wide temperature range. Moreover, the circuitry used by Dell'Acqua et al. requires two thermistor temperature sensing elements to achieve both offset and gauge compensation, and is costly to construct.